The effective electromechanical coupling coefficient of piezoelectric thin-film resonators

Abstract

The input electric impedance Zin for a three-layer thin-film bulk acoustic wave resonator is derived by transfer matrix method to describe the resonator behavior. Based on the impedance spectra, the effective coupling coefficient keff2 of a thin-film resonator can be evaluated with respect to the resonator structure and thin-film properties. The results for both AlN and lead zirconate titanate thin-film resonators reveal that the mechanical Q factor of the thin-film piezoelectric material has a significant effect on the effective coupling coefficient keff2 of the device. keff2 decreases with theincrease of the mechanical quality factor Q. A maximum keff2 value can be obtained at an appropriate thickness ratio of electrode/piezoelectric layers.